It is often desirable to obtain measurements of selected characteristics of sheet materials during manufacture. Although various properties of sheet materials can be detected by off-line laboratory testing, such tests often are not practical because of the time required for sample acquisition and analysis. Also, laboratory testing has the shortcoming that samples obtained for testing may not accurately represent sheet material that has been produced.
To overcome the drawbacks of laboratory testing of sheet materials, various sensor systems have been used for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Typically, on-line sensor devices are operated to periodically traverse, or “scan,” traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet-making operation, cross-directional distances can range up to about 10 meters or more.
A wide variety of scanning sensor devices have been used for on-line measurements of sheet materials. As illustrated in FIGS. 6A and 6B, the scanning sensor system 100 typically includes a stationary frame 102, having a pair of upright end members that stand on a factory floor for supporting a guide member 104 that extends horizontally across a traveling sheet (not shown). A motor driven carriage is mounted to travel on the guide member 104. The carriage is connected to a drive system to be driven back and forth across sheet. The scanning sensor system also includes a scanning head 106 that is mounted on the carriage member. The scanning head 106 contains the detection components. For example, in the case of a spectrometric analyzer, the scanner head can include a source of infrared light, collimating and beam-splitting mirrors, and photosensitive detector.
The scanner head 106 is electronically connected by a wire 108 that is connected to the process controller 110. The scanner head 106 travels back and forth along the cross direction adjacent the traveling sheet being analyzed. In FIG. 6A, the scanner head 106 is at one side of the frame 102 and is moving toward the middle of the frame 102 as shown in FIG. 6B. As is apparent, gravity causes the suspended wire 108 to sag because of the lack of adequate support.
With prior art scanning sensors, the detection components, that are located in the scanner head, are subject to vibrations as the carriage moves back and forth. Moreover, the presence of the relatively heavy detection components in the scanning head makes it more difficult to drive the carriage and to control its speed. These phenomena combined to reduce the precision and accuracy of the detectors. Finally, the inclusion of the detection components within the scanner head makes them less accessible; indeed, the carriage must be stopped and the scanner head disassembled whenever the detection components need to be inspected.